International patent application WO 99/09290 discloses a drill string system provided with a thruster to thrust the drill bit in forward direction, and an anchoring device including radially extendible grippers with actuator pistons to anchor the drill string to the borehole wall during activation of the thruster.
A problem of the known anchoring device is that a separate actuating means is required to bring the pistons (and thereby also the grippers) back to their retracted position after drilling of a further borehole section.